Assuming we use only known/ understood physics, space navies would be boring
and generally weak things, and no fun to join. Even with efficient reaction
drives, their range would be so limited that "manuevers" would look like
slow-motion chess matches. And what would be the point, at the kind of
accellerations we can visualize achieving , no ships would ever get into
shooting range of any worthwhile or understood weapons unless they both
conspired to do so. Low-energy transfer orbits or higher-energy intercepts
would still take days to months to get anywhere useful, then they'd have to
repeat the long trip to ge back, shoudl they survive a battle. And the scales
of movement we're talking about mean flight times would take so long the ships
would have to be autonomous or remotely-piloted, because live crews would run
out of consumables way before the ship could really be effective. Yep, forget
live crew, that M-5 computer is the way to go;-)

Anything resembling a space fleet, until we make some sort of physics
breakthru, will of necessity be strictly an orbital force, most likely a
bombardment component (using something like the THOR system) and an intercept/
CAP element to stop the other guys from doing the same thing. None of these
need live crews. They'd just be sophistcated satellites.